currently if you consume the entire request body this will
terminate and return the ret value that you supplied. But, that
seems wrong, because that will tear down the whole pipeline and
return that value instead of what you really wanted to return.

Perhaps this should return a 'Maybe ByteString' instead so you can
detect when the body ends? But that interfers with using
parseRequest in httpPipe. For now we will just return empty
forever when you get to the end.

Perhaps pipes 2.5 will provide a better solution as it is supposed
to allow you to catch termination of the upstream pipe.